For safety, advanced automobiles are now loaded with air bags which are normally folded and stored in the steering wheel, but upon detection of shocks by collision, can be instantaneously inflated between the wheel and the driver to protect the driver from damages. The air bags are generally formed from nylon fabric by coating it with chloroprene rubber. These air bags undesirably have a short life since chloroprene rubber is less resistant against heat and weathering. The air bags are also required to be flame retardant so that they are resistant against a fire and explosion which can break out upon collision of the car. Since chloroprene rubber is less flame retardant, prior art air bags formed from chloroprene rubber-coated fabric are further coated with a flame retardant silicone on surfaces which might be exposed to flame.
Air bags formed from silicone rubber-coated fabric have the advantage of not requiring a flame retardant coating because well-known flame retardants can be blended in the silicone rubber to render the rubber itself flame retardant. Now air bag manufacturers are more interested in silicone rubber coating compositions for this reason and for their heat resistance and weather resistance.
The air bag using silicone rubber-coated fabric is designed so that it is normally folded within the automotive steering wheel, but instantaneously inflated when explosive gas is injected upon collision. On inflation, the coating applied to the air bag fabric is also instantaneously stretched following the air bag fabric. Thus not only the base fabric, but also the coating should be mechanically strong. To this end, the currently available silicone coating compositions use base polymers of relatively high molecular weight which contain curing agents, flame retardants, adhesion aids, reinforcements and other additives therein.
The coating compositions using high molecular weight base polymers, however, are quite difficult to apply to a coating thickness of 40 to 100 .mu.m normally required for air bags using knife coaters, offset roll coaters, gravure coaters or the like. The coating compositions must be diluted with organic solvents to a sufficient viscosity to allow easy coating to base fabric.
However, the use of organic solvents leads to serious problems including the risk of electrostatic ignition and potential hazard to operators' health since organic solvents can be taken into the body by way of inhalation or skin contact. Cured coatings are formed on base fabric by applying an organic solvent diluted liquid of the coating composition and causing the solvent to evaporate off. The evaporated organic solvent must be recovered at any expense since it causes air pollution.
Among means for changing a coating composition to a solventless system, the simplest way is to reduce the degree of polymerization of a base polymer until the viscosity of a coating composition is reduced to a sufficient level to allow for coating by means of a knife coater or the like. However, a coating composition based on a polymer having a lower degree of polymerization forms a cured coating which is somewhat low in mechanical strength so that the coating can crack upon air bag inflation, allowing hot explosive gas to bleed out therethrough.
If the viscosity of a coating composition is reduced too low, the composition will strike through base fabric, typically plain weave fabric of nylon fibers, resulting in a less smooth surface. If the composition on the fabric rear surface is cured as struck-through and then wound up, the coated fabric gives rise to blocking. Additionally, the struck-through composition will adhere to rolls of the coating machine, adversely affecting operating efficiency and outer appearance. It is thus desired to have a silicone rubber coating composition having an adequate viscosity to allow coating without the need for organic solvent for dilution.
Furthermore, conventional silicone rubber coating compositions tend to form cured coatings which are tacky on their surface, also contributing to a blocking problem.